Stroke is a common health problem throughout the world with 795,000 new strokes occurring each year in the United States alone. Nearly 90% of stroke survivors require therapy, but the majority of patients only achieve poor functional outcome five years after the onset of stroke. Robot-assisted therapy has been proposed as an alternative approach to conventional physiotherapy because robots can easily facilitate the key behavioral signals that drive neural plasticity, which is the basic mechanism underlying improvement in functional outcome after stroke.
A variety of robotic rehabilitation devices have been developed in the last several years for gait therapy. These include the Lokomat, the Gait Trainer, and other devices. However, there have been conflicting results from recent studies about the effectiveness of these devices. Some studies report that, when compared to conventional therapy, robotic rehabilitation achieves greater functional outcome, while others indicate less improvement. Therefore, there is no clear evidence that robotic gait training is superior to conventional physiotherapy for either chronic or sub-acute stoke patients at the present time.
An alternative approach to robotic interventions in gait therapy has been proposed which takes advantage of mechanisms of inter-leg coordination. Considering the cyclic coordination between limbs in human walking, it is hypothesized that there is a mechanism of inter-limb coordination still intact after a hemiplegic stroke that may be utilized to regain functionality of the impaired leg. Utilizing the function of the unimpaired leg to provide therapy to the impaired leg provides several advantages over current rehabilitation protocols. One of the most significant advantages is the safety of the patient, since there is no direct manipulation of the paretic leg. Current robotic rehabilitation devices physically interact and manipulate the paretic leg. Moreover, stimulating a mechanism that is still fully functional may elicit greater functional outcome than in stimulating the impaired mechanism.
However, the sensorimotor control mechanisms of inter-leg coordination are currently not well understood. Various platforms and protocols have been used to investigate bilateral reflex mechanisms during different phases of the gait cycle, with the majority of the experimental protocols focusing on over-ground walking and dropping of the supportive surfaces at distinct gait phases. During posture maintenance, experiments including powerful unilateral displacement of one leg produced bilateral responses both in adults and in healthy human infants. In addition, disturbances in the load feedback as well as the length of specific muscles during walking have been associated with evoked muscular activations of the unperturbed leg. One significant limitation of the previous studies is that the perturbations induced by the previous experiments almost exclusively focus on dropping the walking surface, which causes a disruption in both force and kinematic feedback. When the walking surface is dropped, the ankle kinematics become perturbed in addition to the force feedback that is lost when the foot loses contact with the walking surface. These types of perturbations do not provide any separation of those two feedback mechanisms, and do not allow further in-depth investigation of the role of force and kinematic feedback in gait. In order to answer important questions on inter-leg coordination and sensorimotor control, it is desirable, therefore, to differentiate force and kinematic feedback. Adjustment of the surface stiffness is a unique way to achieve this differentiation, since stepping on a low stiffness platform does not disrupt force feedback (load force remains the same), but affects kinematics.
Moreover, all of the previous studies have failed to separate the mechanisms of gait from those of body weight support and balance. Moreover, most experimental protocols do not consider balance support. As a result, mechanical perturbations and sudden load changes would have likely triggered mechanisms related to body balance and posture. In fact, the latter leads to the activation of inter-limb mechanisms and therefore explains bilateral leg responses. However, little is known whether this effect is exclusively caused by the mechanisms required for body stabilization and balance maintenance, or if it is also brought about from inter-limb coordination and mechanisms of gait. This gap prevents us from fully understanding sensorimotor control of gait, and consequently from engineering effective rehabilitation protocols.
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